1. Field of the Invention
The present invention provides a hold-down type fastener wherein the preload is set without extensive adjustments, and independent of the fastener axial load and wherein the number of fastener components are substantially reduced.
2. Description of the Prior Art
U.S. Pat. No. 4,534,234 discloses a hold-down type fastener for the insertion and extraction of modular units positioned on an airframe shelf. The fastener disclosed in this patent, although providing satisfactory results, uses a substantial number of components adding to the fastener weight. In addition, assembly time is increased with the attendant increase in labor costs. The fastener utilizes a clutching mechanism that requires numerous adjustments during assembly fabrication. In particular, all the components are first assembled together and then positioned on a fixture. By trial and error, the preload is adjusted on a machine recording the preload. Should further adjustment be required, due to unacceptable preload levels, the fastener is then disassembled with everything forward of the adjustable plate component 66 being removed. The adjustable plate 66 is then rotated clockwise or counterclockwise to the desired preload. The fastener is then reassembled, the preload measured and if the selected value is reached, set screws are locked. It should be noted that '234 fastener takes into account the entire axial component assembly in setting the preload which in turn causes the complicated preload setting procedure. Once assembled and used to secure the modular unit, the operator rotates the fastener sleeve clockwise. As the hold-down progresses towards its defined clutching load, the rotational resistance of the sleeve increases steadily until it finally "clutches out." At that point the hold-down provides a fixed, non-adjustable prescribed load against the modular unit. In this case, the tightening torque increases as the hold down approaches the maximum clutch loading.